1. Field of the Invention
This invention relates to a method of controlling an engine in order to suppress the noise produced by vibration of the engine.
2. Description of the Prior Art
In an engine for a vehicle, generally a flywheel is mounted on the rear end of the crankshaft and a pulley for driving a camshaft and auxiliary mechanisms is mounted on the front end of the crankshaft. In such an engine, bending strain is produced in the end portions of the crankshaft due to the combustion pressure which causes the flywheel and the like to vibrate, whereby noise is produced.
In order to avoid the problem, it has been proposed to lower the combustion pressure in the cylinder on the rear side of the engine adjacent to the flywheel so that the bending strain produced in the rear end portion of the crankshaft is minimized, as disclosed, for instance, in Japanese Unexamined Utility Model Publication No. 59(1984)-22985.
In the prior art, the combustion pressure in the cylinder on the rear side of the engine is lowered without taking into account the properties of engine vibration independently from the operating range of the engine. However, the vibration of the engine itself due to combustion in each cylinder includes a first mode component, a second mode component and higher mode components and has inherent properties depending on the strength distribution of the respective components, the frequency which is governed by the engine speed, and the like. Thus, the level and/or the tone of the noise change according to the operating range of the engine depending on the properties of the vibration of the engine, the resonant frequency of the flywheel and the like. Accordingly, when the combustion pressure in the cylinder on the rear side of the engine is lowered by a relatively small rate, the noise suppressing effect can become insufficient and when the combustion pressure in the cylinder on the rear side of the engine is lowered by a relatively large rate in order to avoid this, an unnecessary output loss of the engine can be caused in an operating range where there is substantially no problem of noise.
Further, the vibration of the pulley produced due to the bending strain in the front end portion of the crankshaft also causes the problem of noise. The attempt to suppress the noise at the front side of the engine encounters the same problem. At the same time, the fact that the resonant frequency at the front side of the engine differs from that the rear side of the engine makes the problem more serious.